The color tone of developed silver is a matter of great concern for photographic film makers. The color tone of black and white developed images not only depends on the photographic materials used but also on the grain size, grain thickness, grain structure, grain surface and reflecting power of the developed silver.
It is well known that the warmest tones (yellowish, brownish) can give an unfavorable impression to the observer of the resulting picture image. For this reason a trade requirement of photographic films, in particular for medical X-ray films, is a cold tone (blue-black) in order to make diagnoses easier.
The idea of changing the color tone of a black-and-white image is almost as old as the process of making a black-and-white print itself.
It is known in the art that compounds called toners may be added to emulsion preparations to produce a colder or blacker image after development. References can be found in U.S. Pat. Nos. 4,818,675, 4,201,582, 3,695,880, and 2,512,721, in EP Appl. 271,309, in JP Patent Laid-Open 61/170,739.
It is also known that other chemical ingredients for blue-black image formation can be used with a separate toning bath and or activator bath. Reference can be found in U.S. Pat. Nos. 4,201,582, 3,622,332, 2,192,891 and 2,156,626, Research Disclosure Item 29963, March 1989; Photographic Science & Engineering, Vol 7, No. 2 "Observation on fine structure of developed silver in the presence of added tone modifiers"; C.E.K. Mees, The Theory of the Photographic Process, 1st Edition, p. 568, The Macmillan Co., New York; and A. Rott & E. Weide, Photographic silver halide diffusion processes, pp 61-65, 1972.
The silver image commonly formed during normal development is black, although some silver grains may appear to have warm tone by reflected light. This difference is principally due to a difference in size and in structure of the developed metallic silver particles.
The two pathways to the reduction of silver ions are physical and chemical development. In physical development, which involves a homogeneous chemical reaction, the developing agent reduces a soluble silver salt that is added (or has been make soluble from the silver halide emulsion layer) to the developer, and the formed metallic silver is deposited on the latent image nuclei, resulting in a developed image consisting of compact, rounded particles. In chemical or direct development, which involves a heterogeneous chemical reaction, the silver halide of the grain that has been image-wise exposed is reduced in situ, resulting in a developed image consisting of particles of filamentary structure.
The tight packing of chemically developed filamentary silver ensures the spectral neutrality of this type of silver image and the black color thus appears to be due to multiple scattering and absorption of light. James and Vanselow, in Photographic Science & Engineering, Vol. 1 No. 3, January 1958, "The Influence of the Development Mechanism on the Color and Morphology of Developed Silver" showed that the greater the extent of physical development, the less black was the silver image, the color usually passing to a brownish hue.
The presence of silver halide solvents can dissolve some of the silver halide of the emulsion with the result that the image is formed partly by physical development and is affected by warm tones. In order to reduce as low as possible this physical development it is known to use these compounds in the lowest quantities.
Now, we have surprisingly found that the use of silver halide solvents in combination with organic primary amines dramatically improves the blue-black tone of developed silver and the speed of the black-and-white photographic film.